Manufacture of many electronic components, including flat panel displays, RFID tags, and various sensing applications, relies upon accurately patterning layers of electrically active materials applied to a relatively large substrate. These products are composed of several layers of different patterned materials, where it is important the layers be in specific registration. The reasons for patterning accuracy are twofold. First of all, patterned features must be reproduced across large areas of a substrate while having precise control over their dimensions. Secondly, products built with these features typically are composed of several layers of different, but interacting patterned layers, where it is important that the layers be in specific registration or alignment.
Traditionally, the precise layer alignment required for fabrication of electronic components and devices is accomplished using conventional photolithography. An electrically active layer and a photoresist layer are deposited on a substrate, the position of an existing pattern on the substrate is detected, and an exposure mask is aligned to that existing pattern. The photoresist is exposed, developed, and the electrically active material is etched. Small variations in temperature and humidity in this precise operation may be enough to introduce alignment errors; rigid glass substrates are used with stringent environmental controls to reduce these variations. At the other extreme, conventional printing techniques such as offset lithography, flexography, and gravure printing also apply multiple layers at extremely high speeds, although at substantially lower overlay accuracy.
There is a growing interest in advancing printing technology toward fabrication of thin film electrical components (such as TFTs) on flexible or plastic substrates. These substrates would be mechanically robust, lighter weight, and eventually lead to lower cost manufacturing by enabling roll-to-roll processing. In spite of the potential advantages of flexible substrates, there are many issues affecting the performance and ability to perform alignments of transistor components across typical substrate widths up to one meter or more. In particular, for example, the overlay accuracy achievable using traditional photolithography equipment can be seriously impacted by substitution of a flexible plastic substrate for the rigid glass substrates traditionally employed. Dimensional stability, particularly as the process temperature approaches the transition glass temperature (Tg) of a support, water and solvent swelling, anisotropic distortion, and stress relaxation are all key parameters in which plastic supports are inferior to glass.
Typical fabrication involves sequential deposition and patterning steps. Three types of registration errors are common in these fabrication processes: fixed errors, scale errors, and local misalignments. The fixed error, which refers to a uniform shift of one pattern to another, is typically dominated by the details of the motion control system. Specifically, mechanical tolerances and details of the system integration ultimately dictate how accurately the substrate may be aligned to a mask, or how accurately an integrated print device may be positioned with respect to a registration mark on a moving web. In addition to fixed errors, scale errors may also be substantial. Errors in pattern scale are cumulative across the substrate and arise from support dimensional change, thermal expansion, and angular placement errors of the substrate with the patterning device. Although the motion control system impacts angular placement, pattern scale mismatch is largely driven by the characteristics of the support. Thermal expansion, expansion from humidity or solvent exposure, shrinkage from high temperature exposure, and stress relaxation (creep) during storage of the support all contribute to pattern scale errors. Further, local pattern mismatch arising from nonisotropic deformations may also occur, particularly since the conveyance process involves applying tension. A flexible support used in roll-to-roll manufacturing will typically stretch in the conveyance direction and narrow in width.
There are several approaches to address the registration problem for fabrication of electronics on flexible substrates, but at this point a leading methodology has yet to emerge. Attach/detach technology has been explored by French et al, wherein a flexible substrate is laminated to a rigid carrier and runs through a traditional photolithographic process (I. French et al., “Flexible Displays and Electronics Made in AM-LCD Facilities by the EPLaRTM Process” SID 07 Digest, pp. 1680-1683 (2007)). Unfortunately, these technologies ultimately produce a flexible electronics component only with the cost structure of current glass based processing. US Patent Publication No. 2006/0063351 describes coating the front side and back side of a substrate with one or more resist layers that may be activated simultaneously to impart distinct pattern images within each resist layer. The precoated substrate is inserted between a set of prealigned masks, or alternatively a dual wavelength maskless direct laser writing lithography system is used, to simultaneously expose the front and back sides. Active alignment systems to detect previously existing patterns and compensation schemes for deformation have also been suggested in U.S. Pat. No. 7,100,510 by Brost et al. With this approach, instead of attaining accurate pattern overlay by maintaining tight specs on support dimensional stability and strict environmental control, the motion control system performs multiple alignments per substrate to compensate for distortion. The proposed solution of Brost et al. to adapt traditional printing equipment for active alignment may be viewed as exchanging the lens, mask, and lamp of a modern stepper with an integrated print device. It is difficult to imagine significant equipment cost difference or throughput advantage, particularly if the added task of distortion compensation is included. A fabrication cost advantage would likely come primarily from materials usage savings or removal of expensive vacuum deposition steps.
Another approach, which would potentially enable high speed processing with low capital investment, is to employ a self-aligning fabrication process. In a self-aligning process, a template for the most critical alignments in the desired structure is applied in one step to the substrate and from that point forward alignment of subsequent layers is automatic. Various methods have been described for fabricating self-aligned TFTs. Most of these methods allow self alignment of one layer to another layer, but do not significantly remove the need for very sophisticated alignment steps between several layers. For example, the gate electrode in some a-Si TFT processes is used as a “ask” to protect the channel area from doping and laser annealing of the silicon on either side of the channel region. The concept of self-aligned fabrication can be understood from U.S. Pat. No. 5,391,507 by Kwasnick et al., U.S. Pat. No. 6,338,988 by Andry et al., and US Patent Publication No. 2004/229411 by Battersby.
One published technique offering the potential for a fully self aligned process that eliminates the need for complex registration is Self-Aligned Imprint Lithography (SAIL), as illustrated in U.S. Pat. No. 7,056,834 by Mei et al. In imprint lithography, a variable-thickness resist is prepared on the electronically active layers and a sequencing of chemical etch and materials deposition is matched to controlled erosion of the photoresist to produce TFT structures. There are difficulties with the SAIL process, however. First, a robust nanoimprint technology is needed for webs. Second, the SAIL process requires high accuracy etch depth control, which may not be consistent with a low cost process. Finally, a significant limitation of the SAIL process is that layers produced by the mask cannot be fully independent. As an example, it is particularly challenging to form openings under continuous layers with this approach, an essential element in a matrix backplane design.
Active matrix displays are particularly challenging for electronic component registration and alignment issues, as displays move toward higher resolution, driving the display toward smaller pixel sizes, often with smaller associated electronics. Typically, active matrix displays include liquid crystal displays (LCDs) and electroluminescent displays such as organic light emitting diodes (OLEDs). In an active matrix layout, each pixel is driven by multiple circuit elements such as one, two, or even more transistors, one or more capacitors, and signal lines. For multicolor devices, a pixel is divided into subpixels each with a complete set of circuit elements. For a RGB (red, green, blue) device, each pixel consists of three subpixels, which emit red, green, and blue light. Examples of active matrix OLED devices are provided in U.S. Pat. Nos. 5,550,066; 6,281,634; and 6,456,013; and EP Publication No. 1102317.
In the simplest form, an electroluminescent (EL) device is comprised of an anode for hole injection, a cathode for electron injection, and an electroluminescent media sandwiched between these electrodes to support charge recombination that yields emission of light. In the case where the electroluminescent media is organic, these devices are also commonly referred to as organic light emitting diodes, or OLEDs. A basic organic EL element is described in U.S. Pat. No. 4,356,429 by Tang. Recently, inorganic and inorganic-organic hybrids have become viable technologies for electroluminescent displays and lighting applications, as shown for example in “Tuning the performance of hybrid organic/inorganic quantum dot light-emitting devices,” by Seth Coe-Sullivan et. al. in Organic Electronics, Volume 4, Issues 2-3, September 2003, Pages 123-130.
In order to construct a pixelated EL display useful, for example, in a television, computer monitor, cell phone display, or digital camera display, individual EL elements can be arranged as pixels in a matrix pattern. These pixels can all be made to emit the same color, thereby producing a monochromatic display, or they can be made to produce multiple colors such as a red, green, blue (RGB) display. For present disclosure purposes, a pixel is considered the smallest individual unit which can be independently stimulated to produce light. As such, the red pixel, the green pixel, and the blue pixel are considered as three distinct pixels.
Color EL displays have also recently been described that are constructed so as to have four differently colored pixels. One type of display having four differently colored pixels that are red, green, blue, and white in color is known as an RGBW design. Examples of such four pixel displays are shown in U.S. Pat. No. 6,771,028 by Winters, U.S. Pat No. 7,012,588 by Siwinski, and U.S. Pat. No. 7,230,594 by Miller et al. Such RGBW displays can be constructed using a white organic EL emitting layer with red, green, and blue color filters. The white pixel area is left unfiltered. This design has the advantage of lower power consumption and current density compared to a three-color filtered white-emitting organic EL displays by using the higher efficiency white pixels to produce a portion of gray scale colors.
EL displays are sometimes driven with active matrix circuitry. Active matrix circuitry typically consists of active circuit components such as multiple transistors and one or more capacitors per pixel. Active matrix circuitry components also comprises signal lines such as power lines for supplying electric power to the pixels, data lines for supplying a voltage or current signal to adjust the brightness of the pixels, and select lines for sequentially activating a row of pixels thereby causing the pixels of each row to adjust in brightness in response to the signal of the data lines. The signal lines are typically shared by either a row or a column of pixels. These circuit components permit the pixels to remain illuminated even when the pixels are not being directly addressed.
There is an increase in interest in flexible active matrix displays and other electronics. Consequently, there is a growing interest in depositing and patterning thin film semiconductors, dielectrics, and conductors on flexible substrates, particularly because these supports would be more mechanically robust, lighter weight, and potentially lead to more economical manufacturing by allowing roll-to-roll processing. The present invention facilitates highly accurate patterning of thin films applied to various supports, in a simple and advantageous way, and can solve one or more of the aforesaid problems using various supports including flexible supports that allow flexible active matrix displays.